To me the "IT FACTOR" is someone who is so charismatic that the crowd gets totally into the person. A lot of crowd interaction towards the wrestler. You don't want to take a piss break when the star's music hits.

The Rock has the 'it factor'.
HBK had the 'it factor'.
Eddie Guerrero had the 'it factor'.
Chris Jericho has the 'it factor'.
Stone Cold had the 'it factor'.
CM Punk has the 'it factor'.
Daniel Bryan has the 'it factor'.
Austin Aries has the 'it factor'.

Just some examples there of people who had that little extra special something that puts them a cut above the rest.